Phase variation compensation device, phase variation compensation method and communication device

ABSTRACT

A known pattern comparison type phase difference detection unit (12) detects a phase difference between a known pattern extracted from a received signal and a true value of the known pattern as a first phase difference. M indicates the number of modulation phases in a phase modulation method of the received signal. An M-th power type phase difference detection unit (13) removes a modulation component by raising the received signal to M-th power, and detects phase variation from a modulation phase point used for mapping on a transmission side, as a second phase difference. A phase compensation unit (11) compensates phase variation of the received signal based on an addition result of the first phase difference and the second phase difference.

FIELD

The present disclosure relates to a phase variation compensation device,a phase variation compensation method and a communication devicecompensating phase variation in data communication.

BACKGROUND

In coherent optical communication, a frequency offset (a frequencyerror), which is a frequency difference, occurs between a frequency of areceived signal and a frequency of a local oscillator light source.Further, phase variation such as phase noise occurs in the receivedsignal due to non-linear optical effects, vibration of optical fibers, alaser line width (phase fluctuation) or the like. To cope with this, atechnique has been proposed in which, after removing phase change due tomodulation by raising an input signal to the M-th power, a frequencyerror is detected and fed back to an input side to compensate afrequency offset (see, for example, Patent Literature 1). However,though the frequency offset can be compensated to some extent, a lot ofphase noise remains. Further, in this technique, a phase slip occursunder a condition of a lot of noise or waveform distortion, and there isa possibility that a continuous error occurs. A phase slip compensationcircuit to compensate the phase slip is proposed (see, for example,Patent Literature 2).

Further, a technique is proposed in which a received signal istentatively judged based on a threshold set according to an amplitude,and a difference between an original phase and a phase of the receivedsignal is compensated (see, for example, Patent Literature 3). However,accuracy of phase noise compensation is low only with the tentativejudgment. Therefore, by feeding back a frequency error and a phase errorobtained during calculation to reduce phase variation before thetentative judgment, the accuracy is increased. Further, a technique isproposed in which a known pattern inserted on a transmission side isextracted from a received signal, and a difference between an originalphase and a phase of the received signal is detected to compensate phasenoise (see, for example, Patent Literature 4). However, there is aproblem that phase variation is not sufficiently removed and remains.

CITATION LIST Patent Literature [Patent Literature 1] JP 2015-76727 A[Patent Literature 2] JP 2014-003507 A [Patent Literature 3] JP2014-175991 A [Patent Literature 4] JP 2014-155194 A SUMMARY TechnicalProblem

As described above, in a conventional phase variation compensationdevice and a phase variation compensation method, there is a problemthat, even if phase noise compensation is performed, a phase variationis not sufficiently removed and remains. Further, there is a problemthat, if a phase slip compensation circuit is provided, the circuit iscomplicated.

The present invention has been made to solve the problems as describedabove, and an object is to obtain a phase variation compensation device,a phase variation compensation method and a communication device capableof compensating phase variation with high accuracy, while simplifying acircuit.

Solution to Problem

A phase variation compensation device according to the presentdisclosure includes: first phase difference detection circuitryconfigured to detect a phase difference between a known patternextracted from a received signal and a true value of the known patternas a first phase difference; second phase difference detection circuitryconfigured to remove a modulation component by raising the receivedsignal to M-th power, and detect phase variation from a modulation phasepoint used for mapping on a transmission side, as a second phasedifference, wherein M indicates the number of modulation phases in aphase modulation method of the received signal; and phase compensationcircuitry configured to compensate phase variation of the receivedsignal based on an addition result of the first phase difference and thesecond phase difference.

Advantageous Effects of Invention

According to the present disclosure, it is possible to compensate phasevariation with high accuracy, while simplifying a circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a receiver of a coherent opticalcommunication device according to a first embodiment of the presentinvention.

FIG. 2 is a configuration diagram of a phase variation compensationdevice according to the first embodiment of the present invention.

FIG. 3 is a more detailed configuration diagram of the phase variationcompensation device according to the first embodiment of the presentinvention.

FIG. 4 is a diagram showing a known pattern of a transmitted signal.

FIG. 5 is a diagram illustrating an operation of a phase differencecalculation unit of a known pattern comparison type phase differencedetection unit.

FIG. 6 is a diagram illustrating an operation of the phase variationcompensation device according to the first embodiment of the presentinvention.

FIG. 7 is a configuration diagram showing a modification of the phasevariation compensation device according to the first embodiment of thepresent invention.

FIG. 8 is a configuration diagram showing a phase variation compensationdevice according to a second embodiment of the present invention.

FIG. 9 is a diagram illustrating an operation of the phase variationcompensation device according to the second embodiment of the presentinvention.

DESCRIPTION OF EMBODIMENTS

A phase variation compensation device, a phase variation compensationmethod and a communication device according to the embodiments of thepresent disclosure will be described with reference to the drawings. Thesame components will be denoted by the same symbols, and the repeateddescription thereof may be omitted.

First Embodiment

FIG. 1 is a diagram showing a receiver of a coherent opticalcommunication device according to a first embodiment of the presentinvention. The receiver converts an optical signal received from anoptical fiber 1 to an electrical signal and performs digital processing.

In the receiver, the optical signal received from the optical fiber 1 ispolarized and separated into a horizontally polarized signal and avertically polarized signal by a reception optical module 2. Each of thetwo polarized and separated signals is separated into two orthogonalcomponents and further converted to analog electrical signals. Theseanalog electrical signals are converted to digital signals by an ADconverter 3.

Components after a chromatic dispersion compensation unit 4 correspondto an optical transmission distortion compensation device that performsdigital processing of orthogonal modulation signals outputted from theAD converter 3 as the digital signals to compensate distortion. Here,when the optical signal is propagated through the optical fiber 1, asignal waveform is distorted due to an effect of chromatic dispersion.The chromatic dispersion compensation unit 4 estimates the magnitude ofthe distortion from a received signal and performs compensation.

Further, in optical communication, when a horizontally polarized waveand a vertically polarized wave are combined and transmitted, and areseparated at the time of being received, polarization variation occursdue to an effect of polarization mode dispersion, and a waveform isdistorted. An adaptive equalization unit 5 performs an equalizationprocess for compensating the distortion. Polarization separation isperformed by an optical demodulator in the reception optical module 2first, and polarization separation is performed more completely by theadaptive equalization unit 5. As a compensation process of the adaptiveequalization unit 5, a method in which a long-cycle/known pattern signalor a short-cycle/known pattern signal is inserted on a transmission sideto minimize an error from the signal received, and the like areproposed.

A frequency offset compensation unit 6 compensates a frequency error ofa transmitted/received local signal (a carrier signal). A phasevariation compensation device 7 compensates a residual frequency offsetthat has not been sufficiently compensated by the frequency offsetcompensation unit 6, and residual phase variation, phase noise or aphase slip that has not been sufficiently compensated by the adaptiveequalization unit 5, using a known pattern signal or the like insertedon the transmission side.

A carrier phase reproduction unit 8 compensates phase variation that hasnot been sufficiently removed by the frequency offset compensation unit6 and the phase variation compensation device 7. Specifically, thecarrier phase reproduction unit 8 detects a shift Φ between atentatively judged constellation (signal points) and a receivedconstellation (signal points), and performs correction by performingphase rotation by Φ. The correction by the phase rotation can beperformed by multiplying a signal in complex representation by exp (jΦ).However, when a residual frequency offset, residual phase variation andthe like are almost compensated by the phase variation compensationdevice 7, the carrier phase reproduction unit 8 can be eliminated. Afterthat, an error correction unit 9 performs an error correction process.

FIG. 2 is a configuration diagram of a phase variation compensationdevice according to the first embodiment of the present invention. For areceived signal, frequency offset compensation has been performed beforebeing inputted to the phase variation compensation device 7. Therefore,a received signal inputted to the phase variation compensation device 7includes a residual frequency offset and phase noise that have not beensufficiently compensated by the frequency offset compensation. Note thatit goes without saying that, furthermore, thermal noise is included. Aresidual frequency offset compensation unit 10 compensates the residualfrequency offset. Further, a phase compensation unit 11 compensatesphase variation such as the phase noise.

Though an input signal of the residual frequency offset compensationunit 10 is defined as a received signal here, other output signals arealso referred to as received signals. Therefore, a received signal meansnot only an input signal of the phase variation compensation device 7but also an output signal of the residual frequency offset compensationunit 10. The residual frequency offset compensation unit 10 is notessential. The present invention is also effective for a receivedsignal, the residual frequency offset of which has been compensated inadvance. Note that a received signal is identified as “an output signalof the residual frequency offset compensation unit 10” when necessary.

A known pattern comparison type phase difference detection unit 12detects a phase difference between a known pattern extracted from areceived signal and a true value of the known pattern as a first phasedifference. Since this comparison is phase comparison on 2π, a phasevariation of ±π can be detected. Note that the known pattern is insertedfor every predetermined number of symbols. Therefore, by performinglinear interpolation using the phase difference between the knownpattern extracted from the received signal and the true value of theknown pattern and a phase difference between the next known pattern anda true value, the known pattern comparison type phase differencedetection unit 12 may calculate a phase difference for each symbolbetween both known patterns as a first phase difference. In the case ofremoving effects of noise, an amount of phase variation may becalculated by linear interpolation after extracting phases of adjacentsuccessive known patterns and determining a moving average. In additionto the above-mentioned phase interpolation, it is also possible toperform interpolation processing with the electric field information asa signal from the relationship between the phase and the electric fielddescribed later. However, in this case, it is necessary to perform theinterpolation processing by assuming that the amplitude is constant, forexample, 1.

A received signal phase modulation method is modulation using a phase of2πn/M. Here, M indicates the number of modulation phases (the number ofPSK phases). Though n and M are natural numbers in principle, rationalnumbers are also possible. An M-th power type phase difference detectionunit 13 removes a modulation component by raising a received signal tothe M-th power, and detects phase variation from modulation phase pointsused for mapping on a transmission side, as a second phase difference(not an absolute amount but up to 2π/M). The modulation phase points formapping on the transmission side are, for example, two phase points of 0degrees and 180 degrees in BPSK, and are, for example, four phase pointsof 45 degrees, 135 degrees, 225 degrees and 315 degrees in QPSK. Themodulation component is removed by raising to the M-th power (in thepresent case, raising to the second power and the fourth power), a phaseof each symbol is thus superimposed on an ideal phase that shouldoriginally be taken (one point, and zero during phase synchronization).Furthermore, effects of thermal noise can be reduced by averaging (forexample, moving-averaging) values of a predetermined number of symbols.By this method, the second phase difference can be detected as a phasevariation directly from an ideal phase.

A compensation unit 14 compensates phase variation of a received signalinputted to the M-th power type phase difference detection unit 13 withthe first phase difference. By reducing the phase variation by thecompensation unit 14 as far as possible, it is possible to increaseaccuracy of detecting a phase difference due to remaining phasevariation in the M-th power type phase difference detection unit 13.

A result of addition of the first phase difference detected by the knownpattern comparison type phase difference detection unit 12 and thesecond phase difference detected by the M-th power type phase differencedetection unit 13 is supplied to the residual frequency offsetcompensation unit 10 and the phase compensation unit 11. Beforesupplying the received signal to the known pattern comparison type phasedifference detection unit 12 and the M-th power type phase differencedetection unit 13, the residual frequency offset compensation unit 10compensates a residual frequency offset of the received signal based onthe addition result supplied via a loop filter 15. The phasecompensation unit 11 compensates phase variation of the received signal,such as phase noise, based on the addition result.

FIG. 3 is a more detailed configuration diagram of the phase variationcompensation device according to the first embodiment of the presentinvention. The known pattern comparison type phase difference detectionunit 12 has a known pattern detection unit 16 configured to detect aknown pattern from a received signal, a reference signal memory 17configured to store a true value of the known pattern, and a phasedifference calculation unit 18 configured to calculate a phasedifference between the known pattern extracted from the received signaland the true value as a first phase difference. The known patterndetection unit 16 has a known pattern timing detection unit 19configured to detect a timing of a known pattern from a received signaland a known pattern extraction unit 20 configured to extract the knownpattern from the received signal based on the detected timing.

By performing moving averaging of a predetermined number of symbols fora phase component from which a modulation component has been removed byraising a received signal to the M-th power, extracting only a phaseerror component and dividing the phase error component by M, the M-thpower type phase difference detection unit 13 outputs a phase variationfrom an ideal phase (modulation phase points) as a second phasedifference. Thereby, it is possible to extract a phase variation fromwhich a modulation component has been removed. Note that a thermal noisecomponent can be reduced by moving averaging. For example, in the caseof QPSK, a modulation component becomes 2nπ by a phase being multipliedby four at the time of M=4. By removing 2nπ, only a phase variationcomponent can be extracted. By dividing the phase variation by four, anactual phase variation before the multiplication by four can bedetected. Note that, when a phase variation is large, especially when anamount of phase variation between symbols exceeds ±π/4, a π/2 phaserotation tracking circuit to cause π/2 rotations to be performed in adirection opposite to a variation direction is provided. This is acircuit for, when a phase variation larger than a predetermined valueexists, following the phase variation.

For the M-th power type phase difference detection unit 13, a phaseextraction position is not limited to a position betweenmoving-averaging and a ÷M operation but may be any position afterraising to the M-th power. The same goes for other embodiments describedlater. Further, since a circuit to convert electric field information inoutputting a moving average to phase information at the time ofperforming phase extraction is normally designed, the circuit is notespecially shown.

The first phase difference from the known pattern comparison type phasedifference detection unit 12 is converted to an electric field by aphase/electric field conversion unit 21 and supplied to the compensationunit 14 of an input side of the M-th power type phase differencedetection unit 13. Note that the term “electric field” used in thepresent specification means a complex signal or vector which is amodulation signal shown on polar coordinates. When a phase is indicatedby φ, an electric field is indicated by A exp(jφ)=A(cos φ+j sin φ) (Aindicates an amplitude).

A probability of an amount of phase variation of a signal inputted tothe M-th power type phase difference detection unit 13 exceeding ±π/4 isvery low because an amount of phase variation detected by the knownpattern comparison type phase difference detection unit 12 has beensubtracted in advance. Therefore, in the case of this configuration, theπ/2 phase rotation tracking of the M-th power type phase differencedetection unit 13 can be omitted.

As described later, it is preferable to, when the known patterncomparison type phase difference detection unit 12 is not operating, oran input signal of the M-th power type phase difference detection unit13 is not compensated beforehand due to stop of the compensation unit14, provide the π/2 phase rotation tracking described above because alarge phase variation remains. Note that an operation of the π/2 phaserotation tracking can be controlled by parameter setting or the like.

FIG. 4 is a diagram showing a known pattern of a transmitted signal. Asan example of phase mapping of data, a case where a known pattern is aQPSK pattern will be described. In this case, one symbol of a knownpattern is inserted every predetermined number of data symbols (forexample, every thirty-two symbols) in order of (0,0), (0,1), (1,1),(1,0), . . . on a transmission side. In other cases of multi-valuemodulations, a known pattern can be similarly configured.

FIG. 5 is a diagram illustrating an operation of a phase differencecalculation unit of a known pattern comparison type phase differencedetection unit. On a reception side, the known pattern timing detectionunit 19 detects a timing of a known pattern from a received signal. Theknown pattern extraction unit 20 extracts the known pattern. Bycomparing the extracted known pattern with a reference signal, the phasedifference calculation unit 18 calculates, for each known patternsymbol, each phase difference of the known pattern symbol as a firstphase difference. At this time, a phase difference for each data symbol,between the known pattern symbol and the next known pattern symbol isdetermined by linear interpolation, using a phase difference between thetwo adjacent known pattern symbols. At the time of calculating phases ofthe two adjacent known pattern symbols, a moving average of phases ofsuccessive known patterns may be determined. By this moving average, aphase detection error of the known patterns due to effects of thermalnoise added to the received signal can be reduced.

FIG. 6 is a diagram illustrating an operation of the phase variationcompensation device according to the first embodiment of the presentinvention. In the phase variation compensation device according to thepresent embodiment, operations (1) and (2) below are performed inparallel.

(1) Residual Frequency Offset Compensation by Known Pattern ComparisonType Phase Difference Detection and M-Th Power Type Phase DifferenceDetection.

As shown in FIG. 5, the known pattern comparison type phase differencedetection unit 12 compares a known pattern detected from a receivedsignal with a reference signal and calculates a first phase differencefor each symbol. This first phase difference is supplied to the residualfrequency offset compensation unit 10 via the loop filter 15 and is alsosupplied to the phase compensation unit 11. Furthermore, this firstphase difference is supplied to the compensation unit 14 on an input ofthe M-th power type phase difference detection unit 13 via thephase/electric field conversion unit 21. The compensation unit 14compensates the input signal to the M-th power type phase differencedetection unit 13 with the first phase difference from the known patterncomparison type phase difference detection unit 12 in advance. In otherwords, the M-th power type phase difference detection unit 13 performsprocessing in a state in which the first phase difference from the knownpattern comparison type phase difference detection unit 12 has beenremoved from the input signal in advance. In this case, a phasevariation of the input signal of the M-th power type phase differencedetection unit 13 is at least equal to or smaller than ±π/4, and it ispossible to eliminate the π/2 phase rotation tracking for following alarge phase variation. Actually, the π/2 phase rotation tracking iscontrolled not to operate, by parameter setting. Note that thecompensation unit 14 can be configured by a method of performing complexmultiplication of the received signal represented by an electric fieldvector by a first phase difference converted to an electric fieldvector. Thereby, an amount of phase variation determined by the knownpattern comparison type phase difference detection unit 12 is removedfrom the received signal in advance. Phase/electric field conversion bythe phase/electric field conversion unit 21 is not limited to anarithmetic operation but can also be performed with a memory table.

The M-th power type phase difference detection unit 13 performs raisingto the M-th power, moving-averaging, phase extraction of a phase errorcomponent and a ÷M operation for the received signal compensated withthe first phase difference, and outputs a second phase difference as aphase variation from a phase of an ideal signal from which a modulationcomponent has been removed.

Next, the first phase difference from the known pattern comparison typephase difference detection unit 12 and the second phase difference fromthe M-th power type phase difference detection unit 13 are added up. Aresult of the addition is supplied to the residual frequency offsetcompensation unit 10 via the loop filter 15 and is also supplied to thephase compensation unit 11.

For the result of the addition of the first phase difference and thesecond phase difference, the loop filter 15 detects a phase variationbetween symbols (for example, phase difference of the n-th symbol-phasedifference of the (n−1)th symbol) and performs moving averaging with apredetermined number of symbols. The residual frequency offsetcompensation unit 10 compensates the received signal using the value. Afeedback loop of “received signal detection of first and second phasedifferences loop filter residual frequency offset compensation receivedsignal” is configured. Specifically, the loop filter 15 detects rotationbased on a phase difference for each symbol and corrects the receivedsignal. The received signal is corrected until a phase rotation due to aresidual frequency offset is not detected any more. After that, controlis performed so that the residual frequency offset is removed byfeedback. The loop filter 15 maintains an amount of correction. Thereby,the residual frequency offset that has not been sufficiently compensatedby the frequency offset compensation unit 6 at a previous stage can becompensated.

The known pattern comparison type phase difference detection and theM-th power type phase difference detection are performed for thereceived signal compensated by the residual frequency offsetcompensation unit 10. In a converged state in which the residualfrequency offset is almost compensated, a residual offset component isnot detected by the known pattern comparison type phase differencedetection unit 12 and the M-th power type phase difference detectionunit 13 any more. The loop filter 15 continuously adjusts the amount ofcompensation to the residual frequency offset compensation unit 10 sothat the residual frequency offset component is not detected any more.

Note that, though it has been stated that the loop filter 15 hasfunctions of averaging and holding phase difference information, thesefunctions are not limited to functions of the loop filter 15 but can beimplemented in other circuits. Further, the loop filter 15 can beimplemented with other functions such as a frequency filtering function.The convergence characteristics of the feedback loop can be changed bythe configuration of the loop filter.

(2) Phase Noise Compensation by Known Pattern Comparison Type PhaseDifference Detection and M-Th Power Type Phase Difference Detection.

Phase noise remains in a received signal, the residual frequency offsetof which has been compensated. Therefore, at the time of convergence ofresidual frequency offset compensation, a result of addition of a firstphase difference from the known pattern comparison type phase differencedetection unit 12 and a second phase difference from the M-th power typephase difference detection unit 13 means phase noise. This result ofaddition is also supplied to the phase compensation unit 11, and phasenoise existing in the received signal outputted from the residualfrequency offset compensation unit 10 is compensated by the phasecompensation unit 11. Specifically, the phase compensation unit 11converts phase information, which is the result of addition, to anelectric field vector and performs complex multiplication with anelectric field vector which is an output signal of the residualfrequency offset compensation unit 10. The output of the phasecompensation unit 11 becomes a signal, the residual frequency offset andphase noise of which have been compensated.

As described above, in the present embodiment, a phase variation of areceived signal is compensated, using a first phase difference from theknown pattern comparison type phase difference detection unit 12 and asecond phase difference from the M-th power type phase differencedetection unit 13. Since a phase variation compensation range expands to±π by using the first phase difference, it is possible to omit a phaseslip compensation circuit required in the case of using only the secondphase difference from the M-th power type phase difference detectionunit 13. Therefore, it is possible to compensate phase variation withhigh accuracy, while simplifying a circuit. Further, by compensating aninput of the M-th power type phase difference detection unit 13 with thefirst phase difference in advance, it is possible to improve accuracy ofdetection of the second phase difference by the M-th power type phasedifference detection unit 13.

Note that it is not necessarily required to cause the known patterncomparison type phase difference detection unit 12 and the M-th powertype phase difference detection unit 13 to operate together. If lowpower consumption is required, it is possible to cause only the knownpattern comparison type phase difference detection unit 12 to operate.This can be easily realized by stopping all circuits of the M-th powertype phase difference detection unit 13 and setting the second phasedifference to zero. Therefore, in the case of use in a system where lowpower consumption is important in comparison with transmissionperformance, it is possible to respond only by changing settings.

Further, though description has been made on the case of QPSK modulationin the above example, the configuration of the present embodiment can beapplied to the case of 16QAM. In the case of 16QAM, for four points thatare the nearest to the origin and four points that are the farthest fromthe origin on a constellation, processing can be performed by settingM=4 similarly to the case of QPSK. As for eight points existing atintermediate positions among the points, processing is performed withM=4, and phase shift control is performed. The present embodiment can besimilarly applied to other digital modulation methods.

FIG. 7 is a configuration diagram showing a modification of the phasevariation compensation device according to the first embodiment of thepresent invention. In comparison with the device of FIG. 3, themodification is different in the configuration of the M-th power typephase difference detection unit 13 and the method of feedback from theknown pattern comparison type phase difference detection unit 12 to theM-th power type phase difference detection unit 13, but is the same inthe other components.

In the modification, the compensation unit 14 compensates phasevariation of a received signal after being raised to the M-th power witha first phase difference. Further, according to multiplication of thephase of the input signal by M, the first phase difference inputted fromthe known pattern comparison type phase difference detection unit 12 tothe compensation unit 14 is also multiplied by M. In this case, acompensation circuit of the compensation unit 14 can be configured withan adder. Effects of phase difference feedback from the known patterncomparison type phase difference detection unit 12 and effects of theM-th power type phase difference detection unit 13 are the same as thefirst embodiment.

However, in the case of supplying the first phase difference to thecompensation unit 14, the phase/electric field conversion unit 21 and acomplex multiplication circuit of the compensation unit 14 are requiredin the first embodiment. In comparison, in the modification, an electricfield/phase conversion unit and two M-times multipliers (three-timesadders in the case of M=4) and an adder of the compensation unit 14 arerequired. An adder can significantly reduce a circuit scale and powerconsumption in comparison with a complex multiplier. Therefore, themodification can reduce the circuit scale and power consumption incomparison with the first embodiment. The electric field/phaseconversion unit can be configured with an arithmetic operation etc. or atable similarly to the phase/electric field conversion unit 21.

Note that, though calculation processing about a phase is possible afterthe electric field/phase conversion unit in the modification, it ispossible to, in the moving average processing, perform calculation moreaccurately by performing conversion to electric field information onceand considering amplitude information. Note that it is within a normaldesigning range to perform conversion to phase information again beforeextracting a phase. Further, by using the same conversion table for aplurality of conversions between phase and electric field, the circuitscale can be reduced.

Second Embodiment

FIG. 8 is a configuration diagram showing a phase variation compensationdevice according to a second embodiment of the present invention. Incomparison with the first embodiment, the configuration of the knownpattern comparison type phase difference detection unit 12 is different,but the other components are the same.

In the second embodiment, the known pattern timing detection unit 19detects a timing of a known pattern from a received signal which hasbeen phase-compensated by the phase compensation unit 11. The knownpattern detection unit 16 detects the known pattern from the receivedsignal before being phase-compensated by the phase compensation unit 11,based on this timing. Since a residual frequency offset and phase noiseof the output signal of the phase compensation unit 11 have beencompensated, the known pattern timing can be detected from the outputsignal more accurately. The known pattern extraction unit 20 can performextraction of the known pattern accurately by using this timing. In thiscase, however, it is necessary to consider delay due to the processes ofresidual frequency offset compensation and phase noise compensation.

FIG. 9 is a diagram illustrating an operation of the phase variationcompensation device according to the second embodiment of the presentinvention. The phase variation compensation device according to thepresent embodiment performs operations of steps S1 and S2 below inorder.

Step S1: Residual Frequency Offset Compensation by M-Th Power Type PhaseDifference Detection.

When the phase variation compensation device is started, compensationfor an output signal of the phase compensation unit 11 is not completelyperformed. Therefore, an accurate known pattern timing is not obtainedfrom the known pattern timing detection unit 19 to which the signal isinputted, and it is not possible to perform accurate calculation of afirst phase difference. Therefore, only the M-th power type phasedifference detection unit 13 operates and outputs a second phasedifference. It is possible to, in order to prevent wrong compensation,perform control to stop the operation of the known pattern comparisontype phase difference detection unit 12 during the period.

The M-th power type phase difference detection unit 13 performs raisingto the M-th power, moving averaging, phase extraction of a phase errorcomponent and a ÷M operation for the received signal, and outputs aphase variation from which a modulation component has been removed, as asecond phase difference. Since the known pattern comparison type phasedifference detection unit 12 does not operate, the compensation unit 14cannot compensate a large phase variation in advance, and the largephase variation is also inputted together with the received signal.Therefore, a π/2 phase rotation tracking circuit to respond to the largephase variation is provided at a subsequent stage of the M-th power typephase difference detection unit 13. Actually, an operation of the π/2phase rotation tracking circuit can be controlled by parameter setting.

The second phase difference from the M-th power type phase differencedetection unit 13 is supplied to the residual frequency offsetcompensation unit 10 via the loop filter 15 and is also supplied to thephase compensation unit 11. These operations are similar to those of thefirst embodiment.

It is possible to almost compensate a residual frequency offset andphase noise only by the M-th power type phase difference detection unit13. Thereby, it becomes possible for the known pattern timing detectionunit 19 to obtain a known pattern timing. At that timing, the knownpattern comparison type phase difference detection unit 12 is caused tooperate.

Step S2: Residual Frequency Offset Compensation and Phase NoiseCompensation by Known Pattern Comparison Type Phase Difference Detectionand M-Th Power Type Phase Difference Detection.

An operation in the case of causing the known pattern comparison typephase difference detection unit 12 to operate is similar to that of thefirst embodiment. Note that, in this case, an output of the knownpattern comparison type phase difference detection unit 12 is fed backto an input of the M-th power type phase difference detection unit 13,and an input signal is compensated with a first phase difference, and,therefore, the operation of the π/2 phase rotation tracking circuit atthe subsequent stage of the M-th power type phase difference detectionunit 13 is stopped.

In the present embodiment, detection of a known pattern timing isperformed by a signal for which residual frequency offset compensationand phase noise compensation have been performed. Thereby, extraction ofa known pattern timing and detection of a known pattern can beaccurately performed, and, therefore, it is possible to increaseaccuracy of the first phase difference. Since an input signal of theM-th power type phase difference detection unit 13 can be compensatedwith this first phase difference in advance, accuracy of the secondphase difference can also be improved. Accuracy of the residualfrequency offset compensation and the phase noise compensation based onthese can also be improved. Other components and effects are similar tothose of the first embodiment.

Note that the configuration of the modification of the first embodimentshown in FIG. 7 can be combined with the phase variation compensationdevice of the second embodiment shown in FIG. 8. Thereby, effectssimilar to those of the modification shown before can be obtained.

Note that the chromatic dispersion compensation may be performed byrecording a program for realizing the phase variation compensationmethod according to the first or second embodiment in acomputer-readable recording medium, making a computer system or aprogrammable logic device read the program recorded in the recordingmedium, and executing it. Note that the “computer system” here includesan OS and hardware such as a peripheral device or the like. In addition,the “computer system” also includes a WWW system including a homepageproviding environment (or display environment). Furthermore, the“computer-readable recording medium” is a portable medium such as aflexible disk, a magneto-optical disk, a ROM or a CD-ROM, or a storagedevice such as a hard disk built in the computer system. Further, the“computer-readable recording medium” also includes the one holding theprogram for a fixed period of time, such as a volatile memory (RAM)inside the computer system to be a server or a client in the case thatthe program is transmitted through a network such as the Internet or acommunication channel such as a telephone line. In addition, the programmay be transmitted from the computer system storing the program in thestorage device or the like to another computer system through atransmission medium or a transmission wave in the transmission medium.Here, the “transmission medium” that transmits the program is a mediumhaving a function of transmitting information like the network(communication network) such as the Internet or the communicationchannel (communication line) such as the telephone line. Furthermore,the program may be the one for realizing a part of the above-describedfunction. Further, it may be the one capable of realizing theabove-described function by a combination with the program alreadyrecorded in the computer system, that is, a so-called difference file(difference program).

REFERENCE SIGNS LIST

-   7 phase variation compensation device; 10 residual frequency offset    compensation unit (frequency offset compensation circuitry); 11    phase compensation unit; 12 known pattern comparison type phase    difference detection unit (first phase difference detection    circuitry); 13 M-th power type phase difference detection unit    (second phase difference detection circuitry); 14 compensation unit;    17 reference signal memory; 18 phase difference calculation unit; 19    known pattern timing detection unit; 20 known pattern extraction    unit

1. A phase variation compensation device comprising: first phasedifference detection circuitry configured to detect a phase differencebetween a known pattern extracted from a received signal and a truevalue of the known pattern as a first phase difference; second phasedifference detection circuitry configured to remove a modulationcomponent by raising the received signal to M-th power, and detect phasevariation from a modulation phase point used for mapping on atransmission side, as a second phase difference, wherein M indicates thenumber of modulation phases in a phase modulation method of the receivedsignal; and phase compensation circuitry configured to compensate phasevariation of the received signal based on an addition result of thefirst phase difference and the second phase difference.
 2. The phasevariation compensation device according to claim 1, further comprisingcompensation circuitry configured to compensate phase variation of thereceived signal inputted to the second phase difference detectioncircuitry or the received signal raised to the M-th power, with a firstphase difference.
 3. The phase variation compensation device accordingto claim 1, further comprising frequency offset compensation circuitryconfigured to compensate a frequency offset of the received signal basedon the addition result before supplying the received signal to the firstphase difference detection circuitry and the second phase differencedetection circuitry.
 4. The phase variation compensation deviceaccording to claim 1, wherein the first phase difference detectioncircuitry includes: known pattern timing detection circuitry configuredto detect a timing of the known pattern from the received signal; knownpattern extraction circuitry configured to extract the known patternfrom the received signal based on the timing; a reference signal memoryconfigured to store the true value; and phase difference calculationcircuitry configured to calculate a phase difference between the knownpattern extracted from the received signal and the true value as thefirst phase difference.
 5. The phase variation compensation deviceaccording to claim 4, wherein the known pattern timing detectioncircuitry detects a timing of the known pattern from the received signalphase-compensated by the phase compensation circuitry, and the knownpattern extraction circuitry detects the known pattern from the receivedsignal before being phase-compensated by the phase compensationcircuitry, based on the timing.
 6. The phase variation compensationdevice according to claim 1, wherein by performing linear interpolationusing the phase difference between the known pattern extracted from thereceived signal and the true value and a phase difference between a nextknown pattern and a true value of the next known pattern, the firstphase difference detection circuitry calculates a phase difference foreach symbol between the known pattern and the next known pattern as thefirst phase difference.
 7. A communication device comprising a phasevariation compensation device comprising: first phase differencedetection circuitry configured to detect a phase difference between aknown pattern extracted from a received signal and a true value of theknown pattern as a first phase difference; second phase differencedetection circuitry configured to remove a modulation component byraising the received signal to M-th power, and detect phase variationfrom a modulation phase point used for mapping on a transmission side,as a second phase difference, wherein M indicates the number ofmodulation phases in a phase modulation method of the received signal;and phase compensation circuitry configured to compensate phasevariation of the received signal based on an addition result of thefirst phase difference and the second phase difference.
 8. A phasevariation compensation method performed by a phase variationcompensation device comprising: detecting a phase difference between aknown pattern extracted from a received signal and a true value of theknown pattern as a first phase difference; removing a modulationcomponent by raising the received signal to M-th power, and detectingphase variation from a modulation phase point used for mapping on atransmission side, as a second phase difference, wherein M indicates thenumber of modulation phases in a phase modulation method of the receivedsignal; and compensating phase variation of the received signal based onan addition result of the first phase difference and the second phasedifference.
 9. The phase variation compensation method according toclaim 8, wherein when the phase variation compensation device isstarted, a timing of the known pattern is detected from the receivedsignal whose phase variation is compensated based on only the secondphase difference, and the known pattern is extracted from the receivedsignal before being phase-compensated, based on the timing, and afterthe first phase difference is detected from the known pattern extractedat startup of the phase variation compensation device, a timing of theknown pattern is detected from the received signal whose phase variationis compensated based on the addition result of the first phasedifference and the second phase difference, and the known pattern isextracted from the received signal before being phase-compensated, basedon the timing.